1. Field of the Invention
This invention relates to thin film photovoltaic cells and a method of producing the same, in particular, to cells comprising a CdTe thin coating.
2. Description of the Prior Art
Photovoltaic cells, such as solar cells, convert radiation such as sunlight directly into electric energy. A majority of solar cells nowadays use silicon single crystals. Since the preparation of silicon single crystals requires enormous cost and time, solar cells based on this material are now in use only in specific applications including a lighthouse energy source. Besides, this material operates by an indirect transition mechanism with an essentially low absorption coefficient, thus demanding a relatively large crystal thickness of about 100 microns. Hence, the lower limit of production cost is comparatively high.
In contrast, the use of III-V or II-VI compound semiconductor materials permits a remarkable reduction in the necessary thickness of the photoreceptor by a factor of several tenths or less, thanks to their high absorption coefficients due to their operational mode based on direct transition. A III-V compound semiconductor material is a material consisting of elements of Group III and V of the Periodic Table of Elements such as GaAs. Similarly a II-VI material is one consisting of elements of Group II and VI of the Periodic Table. Moreover, one can produce an energy converter of a polycrystalline morphology enabling further reduction of production costs as well as production time compared to the case of silicon single crystal cells. Among these materials, II-VI compound semiconductors are particularly attractive mainly because of their inexpensive raw materials. Among II-VI semiconductors, CdTe theoretically has the highest efficiency for converting radiation into electric energy.
Homogeneous junction type photovoltaic cells based on CdTe are known which comprise a sintered CdS coating as a transparent electrode, an n-type CdTe layer formed on the CdS coating by means of screen printing, and a p-type surface layer formed on the surface of the n-type CdTe by thermally diffusing p-type impurities in the n-type CdTe. Unfortunately, however, due to the difficulty in uniformly forming a thin p-type layer with a high impurity content, no one has ever succeeded in achieving a sufficiently high conversion efficiency. Besides, the thickness of the CdTe coating prepared by the described method is about 10 microns, which is more than five times thicker than that theoretically required. Since CdTe is a direct transition type semiconductor, the light absorption coefficients .alpha. for photons having energies (n.omega.) exceeding the band gap energy (Eg) increase in proportion to .sqroot.n.omega.. Accordingly, radiations with energies above Eg are completely absorbed by a CdTe layer having a thickness of 2 microns. Thus, thicknesses larger than this value are unnecessary for absorption efficiency. However, far greater difficulties are encountered for such a thin coating of CdTe with a thickness of about 2 microns in the production of a shallow p-n junction with a sufficient degree of uniformity.
On the other hand, Schottky barrier type photovoltaic cells employing a CdTe semiconductor have also been studied. For example, Japanese Patent Application (OPI) No. 119882/1977 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application") or U.S. Pat. No. 4,035,197 discloses photovoltaic cells produced by preheating a CdTe polycrystalline coating in an oxygen-containing atmosphere at a temperature ranging from 250.degree. to 500.degree. C. for 1 to 20 minutes followed by coating a thin barrier forming metal such as gold thereon.
According to the description in the examples of these patents, the CdTe layer was prepared by slicing a hot-pressed product, and one may reasonably conclude this layer has a thickness of about several tens of microns, although definite data is lacking from the disclosures. Thus, they too have unnecessarily large thicknesses in comparison to the theoretical value cited above.
The specifications further point out a desirable effect of the preheating is to improve the open-circuit voltage of the resulting cell. It should be noted that such an effect is only valid for thick hot-pressed products of CdTe. It is unpredictable whether such preheating operation is also effective for CdTe thin layers of 2 microns or less thickness. Actually, when a Schottky barrier type photovoltaic cell comprising a thin CdTe layer provided with a gold barrier was subjected to heat-treatment above 250.degree. C., the open-circuit voltage was experimentally confirmed to be reduced almost to zero as will be shown in the examples later.
Generally speaking, when the thickness of a CdTe layer is reduced to about that theoretically required, the resulting photovoltaic cell proved to exhibit a far poorer performance, in particular, as for open-circuit voltage in comparison to those utilizing hot-pressed products of CdTe.